The present invention relates to an image forming apparatus such as a copying machine or printer in which a toner image formed on an image hearing member through an electrophotographic type or electrostatic recording type is transferred onto an intermediary transfer member and then is transferred onto a recording material or onto a recording material carried on recording material feeding means, and also relates to a belt unit detachably mountable to such an image forming apparatus.
Heretofore, an intermediary transfer type has been used in which a toner image is formed on a drum type electrophotographic photosensitive member (photosensitive drum) (image bearing member) in an electrophotographic type image forming apparatus, and is temporarily transferred (primary transfer) onto an intermediary transfer member, and thereafter, the toner image is transferred onto a recording material such as a sheet of paper from the intermediary transfer member. In the intermediary transfer type, toners images of different colors are sequentially formed by developing devices on the photosensitive drum. The primary transfer step of transferring the toner image onto the intermediary transfer member is repeated to superimpose the different color images on the intermediary transfer member. Then, the toner image comprising the different color images are all together transferred onto the recording material (secondary transfer) to form a full-color image.
As for the intermediary transfer member, an intermediary transfer belt is known which is which is extended around a plurality of rollers along an endless path.
FIG. 9 substantially shows an example of an intermediary transfer belt used in a conventional image forming apparatus.
The intermediary transfer belt 305 used here is resin film or PVDF (polyvinylidene fluoride). Nylon, PET (polyethylene terephthalate), polybarbonate or the like (resistance adjusting treatment has been made if necessary), having a thickness of 100 200 xcexcm and a volume resistivity 1011 1016 xcexa9xc2x7cm, formed into an endless belt. The intermediary transfer belt 305 is stretched around a rear surface roller 316, a driving roller 317, a tension roller 318 and so on.
A primary transfer roller 308 functioning as primary transferring means for transferring the toner image onto the intermediary transfer belt 305 from the photosensitive drum 301, is disposed at a position opposed to the photosensitive drum 301 with the intermediary transfer belt 305 is therebetween. The primary transfer roller 308 is supplied with a primary transfer bias during the transfer operation. The primary transfer roller 308 is a low resistance roller having a volume resistivity not more than 105 xcexa9xc2x7cm.
A secondary transfer roller 309 as secondary transferring means for transferring the toner image onto the recording material P from the intermediary transfer belt 305 is disposed at a position opposed to the rear surface roller 316 with the intermediary transfer belt 305 therebetween. The secondary transfer roller 309 is supplied with a secondary transfer bias during the secondary transfer operation.
As described in the foregoing, by using the thin film as the intermediary transfer belt 305, a large electrostatic capacity such as several 100 to several 1000pF can be assured at the primary transfer nip where the primary transfer roller 308 is opposed to the photosensitive drum 301 through the intermediary transfer belt 305.
In the prior art structure, discharging needles 319 (discharging member) are disposed at a position a predetermined distance away from the primary transfer roller 308 and the intermediary transfer belt 305 downstream of the primary transfer roller 308 with respect to the moving direction (rotational direction) of the intermediary transfer belt 305. The discharging needle 319 has the same potential as the main assembly frame of the image forming apparatus, so that abnormality image in the form of polka-dots under the low temperature and low humidity ambience can be prevented. Therefore, the discharging needle 319 is widely used.
In a known structure, the intermediary transfer belt 305, the primary transfer roller 308, the rear surface roller 316, the driving roller 317, the tension roller 318 and the discharging needle 319 are unified into an intermediary transfer unit or cartridge, which is detachably mountable to the main assembly of the image forming apparatus.
In such a cartridge system, the maintenance operation of apparatus can be, in effect, performed by the users, so that operativity can be improved.
On the other hand, in an image forming apparatus of the electrophotographic type, a so-called in-line type image forming apparatus is known in which the image forming apparatus has a plurality of image bearing members. In such an image forming apparatus, the toner images formed on the respective image bearing members by the respective image forming means (image formation station), are sequentially transferred onto a recording material carried on recording material feeding means for carrying the recording material to the respective the image forming stations. Thereafter, the superposed toner images of the different colors are fixed into a full-color image. As for the recording material feeding means, use is made with an electrostatic attraction conveyer belt (conveyer belt) which electrostatically attracts the recording material.
Substantially similarly to the intermediary transfer unit, in a known system, the conveyer belt, the driving roller around which the conveyer belt is stretched, the tension roller, the transferring means (transfer roller) disposed opposed to image bearing member with the conveyer belt therebetween at each of the image forming stations, an intermediary transfer unit (discharging needle) disposed adjacent the conveyer belt and the transfer means, are formed into a unit or cartridge (recording material feeding unit), which is detachably mountable to the main assembly of the image forming apparatus.
However, in the conventional intermediary transfer unit, there are following problems. During the transportation or storage of the intermediary transfer unit after shipping from the plant and before arrival at the dealers, oil or additive materials may transfer to the intermediary transfer belt from the rubber material of the roller or the like contacted to the intermediary transfer belt due to the unavoidable vibrations and so on with the result of chemical reaction with the intermediary transfer belt (chemical attack). In addition, the intermediary transfer belt may suffer from a creep deformation due to tension, permanent strain due to the contact of transfer roller and a damage due to the discharging needle. They are liable to image defect.
The problems with the intermediary transfer unit may arise with the recording material feeding unit, similarly.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus and a belt unit wherein the creep deformation of the belt, the permanent strain, the chemical attack or the damage of the discharging member can be avoided despite vibrations and so on during the transportation before the start of use.
According to an aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member; a belt for transferring an image from said image bearing member onto a recording material: transferring means for transferring the image from said image bearing member toward said belt, wherein said transferring means presses against said image bearing member through said belt; a tension member for applying tension the belt, wherein said belt is extended around said transferring means and said tension member; releasing means for releasing a tension applied to said belt by said tension member; separating means for separating said transferring means from said belt.
According to another aspect of the present invention, there is provided a belt unit detachably mountable to a main assembly of an image forming apparatus, said unit comprising a belt; transferring means for transferring an image toward said belt; a tension member for applying a tension to said belt, wherein said belt is extended around said transferring means and said tension member, releasing means for releasing a tension applied to said belt by said tension member; separating means for separating said transferring means from said belt.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments or the present invention taken in conjunction with the accompanying drawings.